The present invention relates to a process for preparing acetals of malondialdehyde.
Acetals of malondialdehyde of the formula (I): 
in which R1, R2, and R3 are the same or different alkyl groups, cycloalkyl groups, aralkyl groups or aryl groups, with up to 12 carbon atoms, R4 is an alkyl group, cycloalkyl group, aralkyl group or represents Rxe2x80x94COxe2x80x94 with R=alkyl, cycloalkyl, aralkyl or aryl, and R5 and R6 represent independently of each other H, a cycloalkyl group, an aralkyl group, or an aryl group with up to 12 carbon atoms, are used as intermediates for the preparation of heterocycles such as pyrazols, isoxazols, pyrimidines, 2-aminopyrimidines or pyrimidones. Aside from their use as organic intermediates, they also serve as hardener components for polyvinylalcohol- and polyvinylacetate-films (U.S. Pat. No. 4,655,841).
It is known that the malondialdehyde tetraalkylacetals namely 2-substituted and 2,2-di-substituted malondialdehyde tetraalkylacetals, respectively, can be synthesized by reacting orthoformates with alkylvinyl ethers and substituted alkylvinyl ethers (U.S. Pat. No. 2,527,533) or orthoformates and vinyl esters (U.S. Pat. No. 2,459,076) in the presence of suitable Lewis-acids as the catalyst.
In order to improve the economics of the process, it is desirable to run the reaction in the presence of a heterogeneous catalyst and, optionally, continuously. The use of heterogeneous catalysts for the reaction of orthoformates and vinyl ethers has in fact already been described earlier (U.S. Pat. No. 2,556,321). Also, it has been demonstrated that, for example, methyl vinyl ether and trimethyl orthoformate can be reacted continuously in a tubular reactor using dissolved FeCl3 as the catalyst (EP 0058928).
However, it has been found that thexe2x80x94otherwise advantageousxe2x80x94use of heterogeneous catalysts for the reaction of orthoformates with alkylvinyl ethers leads to several practical problems if conducted conventionally. The most important one is related to catalyst lifetime. Commonly used heterogeneous Lewis acid catalysts, such as acidic montomorillonites and acidic zeolites, as well as otherwise homogeneous Lewis acids, when adsorbed on a suitable support (for example FeCl3 on SiO2) were found to be deactivated after relatively short periods of time. For example, upon the use of montmorillonite K10(copyright) or zeolite. H-BEA 25(copyright) to catalyze the reaction of methyl vinyl ether with trimethyl orthoformate, the catalyst lost significant activity after only 3 hours. The yields dropped from 80% to  less than 60% and the catalyst slowly darkened. Such short catalyst lifetimes are unacceptable under commercial conditions and, in fact, negate the advantages normally associated with the use of heterogeneous catalysts, i. e. reduced catalyst-costs, reduced disposal costs, and environmental friendliness.
In the cases where otherwise homogeneous Lewis acids like FECl3, AICl3 or ZnCl2 were supported on SiO2, Al2O3 or montmorillonites or the commercially available Envirocats(copyright) were used, the catalysts quickly lost their activity, and the products were found to contain significant amounts of the catalyst-cations. This phenomenon is commonly referred to as xe2x80x98leachingxe2x80x99.
It is accordingly an object of the present invention to provide a process that provides the compounds of general formula I from orthoformates and alkylvinyl ethers or orthoformates and vinyl esters in the presence of heterogeneous catalysts without the drawback of unacceptably short catalyst lifetimes.
It is an object of the present invention to provide a process for preparing acetals of malondialdehyde of general formula I: 
in which R1, R2, and R3 are the same or different alkyl groups, cycloalkyl groups, aralkyl groups or aryl groups with up to 12 carbon atoms, R4 is an alkyl group, cycloalkyl group, aralkyl group; aryl group, or represents R7xe2x80x94COxe2x80x94 with R7=alkyl, cycloalkyl, aralkyl, or aryl with up to 12 carbons, and R5 and R6 represent independently of each other H, an alkyl group, cycloalkyl group, aralkyl group or an aryl group with up to 12 carbon atoms, by reacting alkylvinyl ethers of general formula II or vinyl esters of general formula III: 
xe2x80x83and ortho formates of general formula IV: 
xe2x80x83wherein the reaction is carried out in the gas phase using a catalyst.
The present invention provides a process for preparing acetals of malondialdehyde of general formula I: 
in which R1, R2, and R3 are the same or different alkyl groups, cycloalkyl groups, aralkyl groups, or aryl groups with up to 12 carbon atoms, R4 is an alkyl group, cycloalkyl group, aralkyl group, aryl groups, or represents R7xe2x80x94COxe2x80x94 with R7=alkyl, cycloalkyl, aralkyl, aryl with up to 12 carbon atoms, and R5 and R6 represent independently of each other H, an alkyl group, cycloalkyl group, arylalkyl group, or aryl group, with up to 12 carbon atoms which allows for the use of a heterogeneous catalyst under commercial conditions and which leads to superior selectivities.
In a preferred embodiment of the process according to the present invention, R1, R2 and R3 are identical. It is especially preferred that in the compound of formula I, R1, R2, R3 and R4 represent a methyl group. It is also especially preferred that in the compound of general formulas I, R1, R2 and R3 represent a methyl group and R4 represent an acetyl group.
It has been found that improved catalyst-lifetimes of heterogeneous catalysts, as well as superior selectivities, can be achieved in a simple manner and that acetals of malondialdehyde of general formula I are obtained advantageously if alkyl vinylethers of general formula II or a vinylester of general formula III: 
in which R4, R5 and R6 are as defined above and R7 is an alkyl group, cycloalkyl group, aralkyl group, or aryl group, with up to 12 carbon atoms, are reacted with an orthoformate of general formula IV 
xe2x80x83in the gas phase.
Acidic catalysts, preferably strong Lewis acids, such as ZnCl2 on montmorillonite K-306(copyright), ZnCl2 on Al2O3, FeCl3 on SiO2 or Envirocats EPZG(copyright) and EPZ10(copyright) demonstrated relatively high reactivities and, therefore, are preferred.
In order to evaporate the starting materials and especially to avoid condensation of the products of general formula I and/or the starting materials of general formula II respective III and IV within the catalyst bed, it has been found to be advantageous to carry out the reaction under vacuum. In a preferred embodiment according to the present invention, the reaction is carried out at an absolute pressure between about 0.1 mm Hg and about 500 mm Hg, with a pressure of between about 1 mm Hg, and about 50 mm Hg being especially preferred.
While the compounds of general formula II respective III and IV can be reacted in the gas phase at temperatures as low as 20xc2x0 C., it is advantageous to choose temperatures that are significantly higher than the ones that are ideal for the liquid phase. Above 250xc2x0 C., however, the orthoformates of general formula IV were found to decompose to a significant extent, even on very short contact-times. The best yields of the compounds of general formula I were usually achieved in the temperature range of about 40xc2x0 C. to about 150xc2x0 C., and especially in the temperature range of about 70xc2x0 C. to about 100xc2x0 C.
Improved conversions of the starting materials of general formulas II and III are usually achieved if the ortho formates of general formula IV are used in excess, i.e., from about 1.1 to about 3-fold molar excess, preferably about 1.2-fold to about 1.5 fold.
In a preferred embodiment of the process according to the present invention, the reaction is carried out continuously. In practice, it does not matter if a mixture of the compounds of formulas II, III and IV are evaporated, or if individual vapor streams are passed through a common catalyst bed.
It is a special advantage of the process according to the present invention that the separation of the products of general formula I from the starting materials of general formulas II, III and IV can be easily achieved by fractional condensation. In a preferred embodiment, the product of general formula I, unconverted starting materials of general formulas II, III and IV, or a mixture thereof, as well as lower boiling byproducts, such as dialkyl ethers or alkyl formates, are condensed separately by adjustment of the condensation temperatures.
If necessary, the product of general formula I, as well as the recovered starting materials of general formulas II, III and/or IV can then be further purified by standard techniques, preferably by fractional distillation.
In a preferred embodiment according to the present, invention, the recovered starting materials of general formulas II, III and/or IV are recycled back into the process. In doing so, it has been found that a removal of protic contaminants, such as alkanols, is advantageous with respect to obtaining improved yields.
Having described the present invention, reference will now be made to certain examples, which are provided solely for purposes of illustration and are not intended to be limiting.